Spark plugs for use in internal-combustion engines include, for example, an insulator having an axial hole extending in the axial direction, a center electrode inserted in the axial hole, a cylindrical metal shell disposed around the insulator, and a ground electrode connected to the metal shell. There are also known spark plugs that include noble metal tips disposed on the center electrode and the ground electrode at positions where they form a gap where spark discharge occurs to achieve improved wear resistance.
Center electrodes and ground electrodes require high oxidation resistance since they are exposed to high-temperature combustion gases in internal-combustion engines. For example, an alloy that has been proposed for use as a material for a ground electrode contains nickel (Ni) as a major component, 0.50% to less than 1.0% by mass of silicon (Si), 0.2% to 2.0% by mass of aluminum (Al), 12% to 34% by mass of chromium (Cr), 0.03% to 0.2% by mass of at least one element selected from the group consisting of rare earth elements, more than 0% to 20% by mass of iron (Fe), 0.10% by mass or less of carbon (C), and 1.0% by mass or less of manganese (Mn). The total content of silicon (Si) and aluminum (Al) is 0.80% by mass or more and is 1/10 or less of the content of Cr. It is believed that the use of such a composition improves the oxidation resistance of the electrode and thus, for example, inhibits the formation of oxide scale between the electrode and the tip, thereby improving the anti-peeling performance of the tip.
Unfortunately, the related art discussed above is not sufficiently designed to improve the heat conduction performance of the material and can thus lead to, for example, pre-ignition due to electrode overheating.
This specification discloses a technology for reducing the likelihood of pre-ignition induced by spark plugs for use in internal-combustion engines while ensuring sufficient oxidation resistance of electrodes.
The technology disclosed in this specification can be implemented as the following application examples.